Novel food items, such as hazelnuts, are attractive as environmental enrichment, since they form a substrate for both foraging and gnawing behaviour. Hazelnuts are particularly rich in polyunsaturated fat. A hazelnut, weighing 2 g, contains 820 mg crude fat, contributing with 15% of the total intake of dietary fat when feeding with a typical rodent diet. We examined body weight development and serum cholesterol, triglycerides and free fatty acids in male and female Sprague Dawley rats, offered one hazelnut weekly each, for 10 weeks. Control animals were not offered hazelnuts. The animals were five weeks old at the start of the trial, and were fed ad libitum with a standard rodent diet containing 26.6% crude protein, 5.6% crude fat and 8.4% crude fibre. Body weight was measured weekly, and at the start of the trial, after five weeks and after 10 weeks, blood samples were taken from the tail vein for analyses of serum cholesterol, triglycerides and free fatty acids. We found no differences in body weight development between animals receiving hazelnuts and control animals. Males reached a body weight of 429 + 27 g and females 276 + 15 g. No differences in serum cholesterol and free fatty acids were observed. Triglyceride levels were slightly elevated in females receiving hazelnuts (0.98 + 0.43 vs. 1.31 + 0.26 mmol/l). On the basis of this, we concluded that hazelnuts are an acceptable form of environmental enrichment, which does not affect growth and levels of serum cholesterol and free fatty acids.